1. Field of the Invention
The present invention relates to a zinc system-plated steel sheet, plated by continuous steel electroplating using an insoluble anode and, more particularly, to a method of dissolving zinc oxide to supply a sulfuric acid plating liquid with zinc ions consumed in a process of manufacturing a steel sheet plated with a zinc alloy, such as zinc-nickel alloy.
2. Description of the Related Art
Recently, the demand for improvements in corrosion resistance has significantly increased in the fields of motor vehicles and electrical home appliances. Alloy-plated steel sheets such as recently developed zinc-nickel-plated steel sheets and the like are now in markedly increased demand.
To cope with such increasing demands, a high-speed electroplating method using a high current density, to enable high-efficiency production, is adopted. Needless to say, in high-speed electroplating, a method of continuously supplying consumed metallic ions by using an insoluble anode, such as an iridium-system anode, which needs no interchange, is more advantageous than soluble anode methods in which anode interchange must be frequently made.
In such continuous electroplating, a mineral acid electrolytic solution such as sulfuric acid is ordinarily used as a plating liquid, and methods of supplying zinc ions to a plating liquid are generally grouped into methods of dissolving metal zinc by bringing it into direct contact with a plating liquid and methods of dissolving a zinc compound such as zinc oxide.
Ordinarily, if an insoluble anode is used, it is necessary to supply a large amount of zinc ions to a plating bath, and a method of dissolving metal zinc is adopted because it is low-cost.
However, metal zinc has a low dissolving rate in sulfuric acid plating liquids. Therefore, it is necessary to use an additional method of increasing the contact area with the plating liquid by roughening the surface of the zinc plate or using a fine zinc powder, or using a method of increasing the residence time. Working the metal zinc into a fine powder, however, increases the manufacturing cost, and a considerably large equipment cost is required to increase the residence time.
Therefore, a method has also been proposed in which low-priced metal zinc and a zinc compound such as zinc oxide having a high dissolving rate are used in combination and two comparatively small dissolution baths at upstream and downstream positions. However, if zinc-nickel alloy electroplating is performed by using this method, a substitutional precipitation phenomenon occurs between the zinc source and nickel sulfate dissolved in the plating liquid circulatory which is used to cause the nickel (which is a "precious" metal) to precipitate. This occurs as long as metal zinc is used as the zinc source. Then, a need for recovering and reusing high-priced metal nickel arises. Thus, this method is not always advantageous in terms of cost.
Methods of dissolving a zinc compound free from substitutional precipitation and having a comparatively high dissolving rate in a plating liquid have therefore been proposed.
Japanese Patent Laid-Open No. 85531/1975 discloses a method of using one of zinc hydroxide and zinc carbonate, a mixture of these compounds or a mixture of these compounds and zinc oxide, and Japanese Patent Laid-Open No. 87689/1981 discloses a method of dissolving zinc oxide or basic salt of zinc. However, if a zinc compound such as zinc oxide is directly thrown into an acid plating liquid, agglomeration is liable to occur due to heat of reaction or the like caused on the surfaces of powder particles, and the agglomerated compound does not dissolve.
Japanese Patent Publication No. 13639/1983 then discloses an improved method in view of this problem, i.e., a method of previously dispersing zinc oxide or basic salt in water before throwing it into a plating bath. This method prevents agglomeration but entails a new problem of requiring a high-speed agitator (homogenizer) for dispersion treatment and a water removing means such as an evaporator, since an excessively large amount of water is supplied to the plating liquid.
The inventors of the present invention have studied various methods for efficiently dissolving zinc oxide in a sulfuric acid plating liquid circulated between a plating bath and a dissolution bath, and thereby discovered that, in the dissolution bath, depending upon the pH range thereof, iron ions eluted from a steel sheet precipitate as an iron compound of an unknown structure, and that such a precipitate forms a film on the surface of zinc oxide to prevent dissolution of zinc oxide.
As described above, no satisfactory method has presently been discovered for supplying zinc ions in continuously electroplating using an insoluble anode.